Determination of Fracture Modes in Novel Aluminum-Steel Dissimilar Resistance Spot Welds

With increasing demands to improve vehicle fuel economy, multi-material body structures are increasingly utilized in the automotive industry for structural lightweighting purposes. These multi-material structures pose challenges in dissimilar material joining, particularly aluminum to steel. General Motors (GM) developed a new resistance spot welding technique using a multi-ring domed electrode and multiple solidification weld schedules to address these challenges. In aluminum-steel resistance spot welds (RSWs), an iron-aluminum intermetallic compound (IMC) layer is formed at the interface and its strength affects tensile shear specimen fracture modes, i.e. interfacial versus pull out fracture. Based upon the experimental heat affected zone (HAZ) and IMC shear strengths using a new mini-shear test specimen, it was observed that it was not suitable to use the critical weld nugget diameter of 4√t recommended by the American Welding Society (AWS) to determine the fracture modes of these unique aluminum-steel spot welds. In the present study, a new formula considering the shear strength of intermetallic layer in aluminum to steel RSWs is derived to calculate a critical aluminum-steel weld nugget diameter based upon experimental results. The calculated critical weld nugget diameters were then compared with experimental results to predict fracture modes for aluminum-steel stack-ups with different sheet thicknesses.